Althea V. Moorhead

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13066]
NASA Meteoroid Environment Office, Marshall Space Flight Center, Huntsville, Alabama, USA
Published by arrangement with John Wiley & Sons

Debiasing the velocity distribution of meteors observed by the Canadian Meteor Orbit Radar (CMOR) yields a distribution with large numbers of slow meteors. The distribution also contains significant numbers of hyperbolic meteors, in conflict with the expectation that interstellar meteors should be rare. In Moorhead et al. (2017a), we noted that measurement uncertainties were possibly smoothing the speed distribution and redistributing meteors to the extreme ends of the speed distribution. In this report, we use techniques analogous to image sharpening to remove the blurring caused by measurement uncertainties. The deconvolved speed distribution appears to have no meteors slower than 14 km s⁻¹ and none faster than 74 km s⁻¹. The result is to substantially raise the characteristic velocity of incoming meteoroids from 12.9 to 20.0 km s⁻¹.

Jens Hopp^1,2,*, Natalie Schröter¹, Olga Pravdivtseva³, Hans-Peter Meyer¹, Mario Trieloff^1,2 and Ulrich Ott^1,4,5

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13062]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Heidelberg, Germany
³McDonnell Center for the Space Sciences and Physics Department of Washington University, Saint Louis, Missouri, USA
⁴MTA Atomki, Debrecen, Hungary
⁵Max-Planck-Institut für Chemie, Mainz, Germany
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of ⁴⁰Ar/³⁹Ar and ¹²⁹I-¹²⁹Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. ²¹Ne and ³⁸Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This ³⁸Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our ⁴⁰Ar/³⁹Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable ⁴⁰Ar/³⁹Ar age could be determined. The integrated age strongly depends on the choice of an initial ⁴⁰Ar/³⁶Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air ⁴⁰Ar/³⁶Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of ¹²⁹I-¹²⁹Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest ¹²⁹Xe*/¹²⁸Xe_I ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated ¹²⁹Xe/¹³²Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.

E. B. Bierhaus^1,*, A. S. McEwen², S. J. Robbins³, K. N. Singer³, L. Dones³, M. R. Kirchoff³ and J.-P. Williams⁴

Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13057]
¹Lockheed Martin Space, Denver, Colorado, USA
²University of Arizona, Tucson, Arizona, USA
³Southwest Research Institute, Boulder, Colorado, USA
⁴University of California, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

We review the secondary-crater research over the past decade, and provide new analyses and simulations that are the first to model an accumulation of a combined primary-plus-secondary crater population as discrete cratering events. We develop the secondary populations by using scaling laws to generate ejecta fragments, integrating the trajectories of individual ejecta fragments, noting the location and velocity at impact, and using scaling laws to estimate secondary-crater diameters given the impact conditions. We also explore the relationship between the impactor size–frequency distribution (SFD) and the resulting secondary-crater SFD. Our results from these analyses indicate that the “secondary effect” varies from surface to surface and that no single conclusion applies across the solar system nor at any given moment in time—rather, there is a spectrum of outcomes both spatially and temporally, dependent upon target parameters and the impacting population. Surface gravity and escape speed define the spatial distribution of secondaries. A shallow-sloped impactor SFD will cause proportionally more secondaries than a steeper-sloped SFD. Accounting for the driving factors that define the magnitude and spatial distribution of secondaries is essential to determine the relative population of secondary craters, and their effect on derived surface ages.